Radiation therapy (radiotherapy) refers to the treatment of diseases, primarily but not limited to the treatment of tumors such as cancer, with radiation. Radiotherapy is used to destroy malignant or unwanted tissue without causing excessive damage to the nearby healthy tissues.
Ionizing radiation can be used to selectively destroy cancerous cells contained within healthy tissue. Malignant cells are normally more radiosensitive than healthy cells. Therefore, by applying radiation of the correct amount over the ideal time period, it is possible to destroy all of the undesired cancer cells while saving or minimizing damage to the healthy tissue. For many decades, localized cancer has often been cured by the application of a carefully determined quantity of ionizing radiation during an appropriate period of time. Various methods have been developed for irradiating cancerous tissue while minimizing damage to the nearby healthy tissue. Such methods include the use of high-energy radiation beams from linear accelerators and other devices designed for use in external beam radiotherapy.
Another method of radiotherapy includes brachytherapy. Here, substances in the form of seeds, needles, wires or catheters are implanted permanently or temporarily directly into/near the cancerous tumor. Radioactive materials used include radon, radium and iridium-192. More recently, the radioactive isotopes cesium-131, iodine-125 and palladium-103 have been used. Examples are described in U.S. Pat. Nos. 3,351,049; 4,323,055; and 4,784,116, which are incorporated herein by reference.
During the last 30 years, over 100 papers (both scientific and lay publications) have been published on the use of iodine-125 and palladium-103 in treating slow growth prostate cancer. Despite the demonstrated success of iodine-125 and palladium-103, there are certain disadvantages and limitations in their use. While the total dose can be controlled by the quantity and spacing of the seeds, the dose rate is set by the half-life of the radioisotope (60 days for I-125 and 17 days for Pd-103). For use in faster growing tumors, the radiation should be delivered to the cancerous cells at a faster, more uniform rate, while simultaneously preserving all of the advantages of using a soft X-ray emitting radioisotope. Such cancers are those found in the brain, lung, pancreas, prostate and other tissues.
The low energy X-ray seeds of I-125 and Pd-103 have been used successfully in the treatment of cancer. Methods for encapsulation of these radioactive isotopes as seeds or capsules for interstitial brachytherapy are described in U.S. Pat. Nos. 1,753,287; 3,351,049; 4,323,055; 4,702,228; 4,784,116; 4,891,165; 4,994,013; 5,163,896; 5,405,309 and 6,099,458, and are incorporated herein by reference.
Some of the above-referenced seeds suffer from a number of disadvantages and limitations. They include: (a) the lower energy of the X-ray from Pd-103 (20 key); (b) the half-life of the I-125 seed (60-days) is typically too long to permit its use as a permanent implant in anything other than slow growing tumors; (c) the use of a silver wire marker incorporated into I-125 seeds (U.S. Pat. No. 4,323,005) has a large unwanted amount of characteristic low energy (<10 keV) silver K-X-rays; and (d) seeds with various internal components with non-optimized geometrics, which require a greater amount of isotope to compensate for the non-uniform dose pattern surrounding the seed.
The use of sealed radioactive sources became so widespread that standards used for production were established for radiation protection. Leakage test methods for sealed radioactive sources continue to be refined (ISO 9978:1992(E) and ISO 2919:1999(E)). In order to meet these standards, the sealed seeds must be exposed to various environmental stresses while remaining completely sealed. The most difficult test to pass is the impact test, which requires that the metal seed body be placed on a steel anvil, and that a 2.54 cm diameter, 50 gram weight be dropped on the seed from a height of 1 meter. Even though the typical titanium cased seed can is then flattened, there must be no leakage from the metal seed body. To pass this stringent test, seed manufacturers (e.g., in constructions such as in prior art such as U.S. Pat. Nos. 4,702,228; 4,784,116; 4,891,165; 5,405,309, etc.) incorporate thick end welds, thick end caps and/or massive internal components in order to retain the original cylindrical shape. However, these characteristics can result in non-uniform dosimetric patterns around the seed due to the self-absorption of the radiation. This non-isotropic dose increases the probability that cancer cells residing in this shadowed low dose area will survive and give rise to a return of the cancer at a later date.
Several prior art seed designs (U.S. Pat. Nos. 3,351,049; 4,994,013; and 5,163,896) suggest the use of plastics, a multitude of ion exchange resin types (nylon, cellulose ester or acetate binders, or rubber) to capture and hold the radioactive material. With increasing concentrations of radioisotopes, unacceptable gas pressure (e.g., H2, CO, CO2, CH4, etc.) may be generated by the autoradiolysis of organic materials within the cavity of the seed. Release of radioactive material due to a breach in the seed can by over-pressurization is a risk.
While the invention was motivated in addressing at least some of the above issues, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.